Rook's crashcollector pod posts entries to the ceph cluster when a crash occurs.
Over time the number cluster may hold crash entries needlessly.
To clean up old crash entries, this PR adds a field to the ceph cluster CR for the user to specify the number of days a crash entry should be kept for.
Providing a value for the field keepXDays creates a cronjob that runs every day at midnight, calling "ceph crash prune <keepXDays>".

Closes: https://github.com/rook/rook/issues/6332

Signed-off-by: Renan Campos <rcampos@redhat.com>